Very high energy γ-rays probe the long-standing mystery of the origin of cosmic rays. Produced in the interactions of accelerated particles in astrophysical objects, they can be used to image cosmic ...particle accelerators. A first sensitive survey of the inner part of the Milky Way with the High Energy Stereoscopic System (HESS) reveals a population of eight previously unknown firmly detected sources of very high energy γ-rays. At least two have no known radio or x-ray counterpart and may be representative of a new class of "dark" nucleonic cosmic ray sources.
We report the discovery of very-high-energy (VHE) g-ray emission of the binary system PSR B 1259-63/SS 2883 of a radio pulsar orbiting a massive, luminous Be star in a highly eccentric orbit. The ...observations around the 2004 periastron passage of the pulsar were performed with the four 13 m Cherenkov telescopes of the HESS experiment, recently installed in Namibia and in full operation since December 2003. Between February and June 2004, a g-ray signal from the binary system was detected with a total significance above 13s. The flux was found to vary significantly on timescales of days which makes PSR B 1259-63 the first variable galactic source of VHE g-rays observed so far. Strong emission signals were observed in pre- and post-periastron phases with a flux minimum around periastron, followed by a gradual flux decrease in the months after. The measured time-averaged energy spectrum above a mean threshold energy of 380 GeV can be fitted by a simple power law F0(E/1 TeV)G with a photon index G = 2.7 c 0.2stat c 0.2sys and flux normalisation F0 = (1.3 c 0.1stat c 0.3sys) x 10-12 TeV-1 cm-2 s-1. This detection of VHE g-rays provides unambiguous evidence for particle acceleration to multi-TeV energies in the binary system. In combination with coeval observations of the X-ray synchrotron emission by the RXTE and INTEGRAL instruments, and assuming the VHE g-ray emission to be produced by the inverse Compton mechanism, the magnetic field strength can be directly estimated to be of the order of 1 G.
Multiwavelength observations of supernova remnants can be explained within the framework of the diffusive shock acceleration theory, which allows effective conversion of the explosion energy into ...cosmic rays. Although models of nonlinear shocks describe the nonthermal component of the emission reasonably well, certain issues, including the heating of the thermal plasma and the related X-ray emission, still remain open. We discuss how the evolution and structure of supernova remnants is affected by strong particle acceleration at the forward shock. We use analytical estimates combined with detailed discussion of the physical processes. The overall dynamics is shown to be relatively insensitive to the amount of particle acceleration, but the post-shock gas temperature can be reduced to a small multiple of the ambient temperature (even as low as six times) with a weak dependence on the shock speed. This is in marked contrast to models with no particle acceleration where the post-shock temperature is insensitive to the ambient temperature and is determined by the square of the shock speed. It thus appears to be possible to effectively suppress thermal X-ray emission from remnants by strong particle acceleration. This might provide a clue to understanding the lack of thermal X-rays from the TeV bright supernova remnant RX J1713.7-3946.
The energetic electromagnetic eruptions observed during the prompt phase of gamma-ray bursts are attributed to synchrotron emissions. The internal shocks moving through the ultrarelativistic jet, ...which is ejected by an imploding supermassive star, are the likely source of this radiation. Synchrotron emissions at the observed strength require the simultaneous presence of powerful magnetic fields and highly relativistic electrons. We explore with 1-D and 3-D relativistic particle-in-cell simulations the transition layer of a shock, which evolves out of the collision of two plasma clouds at a speed 0.9c and in the presence of a quasi-parallel magnetic field. The cloud densities vary by a factor of 10. The number densities of ions and electrons in each cloud, which have the mass ratio 250, are equal. The peak Lorentz factor of the electrons is determined in the 1-D simulation, as well as the orientation and the strength of the magnetic field at the boundary of the two colliding clouds. The relativistic masses of the electrons and ions close to the shock transition layer are comparable as in previous work. The 3-D simulation shows rapid and strong plasma filamentation behind the transient precursor. The magnetic field component orthogonal to the initial field direction is amplified in both simulations to values that exceed those expected from the shock compression by over an order of magnitude. The forming shock is quasi-perpendicular due to this amplification. The simultaneous presence of highly relativistic electrons and strong magnetic fields will give rise to significant synchrotron emissions.
Aims.Search for Very High Energy γ-ray emission in the Kookaburra complex through observations with the HESS array. Methods.Stereoscopic imaging of Cherenkov light emission of the γ-ray showers in ...the atmosphere is used for the reconstruction and selection of the events to search for γ-ray signals. Their spectrum is derived by a forward-folding maximum likelihood fit. Results.Two extended γ-ray sources with an angular (68%) radius of $3.3{-}3.4$´ are discovered at high (>13σ) statistical significance: HESS J1420-607 and HESS J1418-609. They exhibit a flux above 1 TeV of ($2.97 \pm 0.18_{\rm stat} \pm 0.60_{\rm sys}) \times 10^{-12}$ and ($2.17 \pm 0.17_{\rm stat} \pm 0.43_{\rm sys}) \times 10^{-12}$ cm-2 s-1, respectively, and similar hard photon indices ~2.2. Multi-wavelength comparisons show spatial coincidence with the wings of the Kookaburra. Two pulsar wind nebulæ candidates, K3/PSR J1420-6048 and the Rabbit, lie on the edge of the HESS sources. Conclusions. The two new sources confirm the non-thermal nature of at least parts of the two radio wings which overlap with the γ-ray emission and establish their connection with the two X-ray pulsar wind nebulæ candidates. Given the large point spread function of EGRET, the unidentified source(s) 3EG J1420-6038/GeV J1417-6100 could possibly be related to either or both HESS sources. The most likely explanation for the Very High Energy γ-rays discovered by HESS is inverse Compton emission of accelerated electrons on the Cosmic Microwave Background near the two candidate pulsar wind nebulæ, K3/PSR J1420-6048 and the Rabbit. Two scenarios which could lead to the observed large (~10 pc) offset-nebula type morphologies are briefly discussed.
Aims. PKS 0548–322 (z = 0.069) is a “high-frequency-peaked” BL Lac object and a candidate very high energy (VHE, E > 100 GeV) γ-ray emitter, due to its high X-ray and radio flux. Observations at the ...VHE band provide insights into the origin of very energetic particles present in this source and the radiation processes at work. Methods. We report observations made between October 2004 and January 2008 with the HESS array, a four imaging atmospheric-Cherenkov telescopes. Contemporaneous UV and X-ray observations with the Swift satellite in November 2006 are also reported. Results. PKS 0548-322 is detected for the first time in the VHE band with HESS We measure an excess of 216 γ-rays corresponding to a significance of 5.6 standard deviations. The photon spectrum of the source is described by a power-law, with a photon index of Γ = 2.86 ± 0.34stat ± 0.10sys. The integral flux above 200 GeV is ~1.3% of the flux of the Crab Nebula, and is consistent with being constant in time. Contemporaneous Swift/XRT observations reveal an X-ray flux between 2 and 10 keV of F2-10 keV = (2.3±0.2)×10-11 erg cm-2 s-1, an intermediate intensity state with respect to previous observations. The spectral energy distribution can be reproduced using a simple one-zone synchrotron self Compton model, with parameters similar those observed for other sources of this type.
Observations of the globular cluster 47 Tucanae (NGC 104), which contains at least 23 millisecond pulsars, were performed with the H.E.S.S. telescope system. The observations lead to an upper limit ...of F(E>800 GeV) < 6.7e-13 / cm^2 s on the integral gamma-ray photon flux from 47 Tucanae. Considering millisecond pulsars as the unique potential source of gamma-rays in the globular cluster, constraints based on emission models are derived: on the magnetic field in the average pulsar nebula and on the conversion efficiency of spin-down power to gamma-ray photons or to relativistic leptons.
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